Method of making steel reinforcing bars for concrete structures



p 10, 1957 A. FRZKJAER-JENSEN 2,805,470

METHOD OF MAKING STEEL REINFORCING BARS,FOR CONCRETE STRUCTURES Filed May 5, 1955 3 Sheets-Sheet 1 Fig.7 H92 H93 H94,

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100d 95d 20:! 75d 7272a 70d Msfmg INVENTOK p 1957 A. FRZKJAER-JENSEN 2,805,470 METHOD OF MAKING STEEL REINFORCING BARS FOR CONCRETE STRUCTURES Filed May 5, 1955 3 Sheets-Sheet 2 r 311- 6 39 E I I 4 3 "vi I 4 2 I I/ 100d 75d 20a, 150 12-/2d 10d 754527779 Sireidnhg A 50 l 4 L52 F197 141d MV M INVENTOR.

Se t. 10, 1957 A. FRQKJAER-JENSEN 2,805,470

METHOD OF MAKING STEEL REINFORCING V BARS FOR CONCRETE STRUCTURES Filed May 5, 1955 5 Sheets-Sheet 3 A SfreM/ng 0/0 55 100 1551 20d 45a 12% 70d Tm'sf/hg Wag MM WENT"? RGETHGD 9F PAAKING STEEL REDIFORCENG BARS FOR CGNCRETE STRUCTURES Axel Frokjaer-lensen, Copenhagen-Charlotteniund,

- Denmark Application May 5, 1955 Serial No. 556,275 1 Claim. (Cl. 29-155) This invention relates to a method of making steel reinforcing bars for concrete structures.

It is known that the strength of reinforcing bars can be increased by cold working, e. g. by stretching or by twisting or by stretch-twisting, i. e. twisting combined with a slight axial load for the purpose of avoiding loops, buckles etc. on the bar if it is subjected to twisting between 2 fixed twisting heads, see Austrian Patent No. 154,017 and U. S. Patent No. 1,692,505.

Furthermore, methods are known according to which the bar is first subjected to cold-rolling and then subjected to twisting or stretch-twisting, see U. S. Patent No. 2,260,779. Furthermore, methods are known according to which two or more bars are twisted together, either without previous cold-working, see U. S. Patent No. 1,692,505 or after the individual bars have previously been subjected to cold-rolling or twisting or stretch-twisting or combinations thereof, see U. S. Patent No. 2,260,779.

In practice the twisting-together of two bars has'been used to a certain extent but has given way to the stretchtwisting of a single bar, which method has been distributed especially on the European Continent. The other methods have never gained any distribution in practice.

Regarding the stretch-twisting it is acknowledged that the axial tensile load shall be below the yield stress as otherwise unfavourable and uncontrollable efiects might arise, cf., German Patent No. 690,484 according to which the stretch-twisting is still carried out with a tensile load below the yield stress. The bars produced in this way are known on the European Continent under the name oflorsteel.

The reason why it is necessary to subject the bar to a slight axial tensile load is that twisting of a round or substantially round bar causes an extension of this, an extension which certainly amounts to only a fraction of one percent but which is sufficient for the forming of loops, buckles etc. and for can 'ng an uneven strengthening of the bar if this is not simultaneously subjected to a slight axial tensile load. So this tensile load is strengthening only as far as it prevents some places of the bar from being weaker than others, whereas the real strengthening is caused by the twisting.

According to my invention the bar is subjected to twisting and an axial tensile stress of such a magnitude that the longitudinal strain after the cold-working is greater than the strain throughout the tensile yield range of the hot-rolled bar.

A'permanent'l ngitudinal strain of this magnitude is critical because only such strain results in a substantial strengthening in addition to the strengthening caused by the twisting' The said stretching and twisting may be effected simultaneously or successively. I 1

It might be expected that the bar would be brittle when, in addition tov twisting, it had been subjected to a tensile stress of such a magnitude as defined above. As explained below, it appears, however, from tests that this is not the case.

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Patented Sept. 10, 1957 bar can be carried on to a greater extent than hitherto possible and therefore also to increased strength properties, than so far obtained.

A still further advantage of the method according to the invention consists therein that different degrees of stretching and twisting can be combined in such a way that any desired form of the stress-strain diagram of the coldworked bar can be obtained thus making it possible to satisfy in an almost perfect way all the requirements of the local ofl'icial standards.

For the better understanding of the invention reference is now made to the accompanying drawing, which exhibit four stress-strain diagrams.

The Figures 1-4 exhibit four stress-strain diagramsof which the first two are taken from very useful materials and the last two are taken from materials which would be very dangerous to use in reinforced concrete structures.

Figures 5-8 are diagrams in which some characteristic cold treatments according to the invention are indicated.

Fig. 1 is a normal mild steel,

Fig. 2 is a steel bar treated according to the present invention, i. c. it is cold-worked by a combination of plastic stretching and plastic twisting in accordance with the method of this invention,

Fig. 3 is a steel bar subjected to an incorrect cold-working without stretching. The uniformly distributed elongation (i. e. the elongational maximum stress) is very small, although the total elongation at fracture is sufficient to satisfy standards in most countries. The proportion between yield point 5.2 and maximum load is very near 100%. The steel therefore, is dangerous to use.

Fig. 4 designates-a diagram for a steel bar which has been cold-worked by a process,- the last step of which is a stretching. A real yield point has reappeared (after one months strain aging) very near the value of the maximum stress. The uniformly distributed elongation is sufiicient, but there is no guarantee against collapse, if the structure should be overloaded. Because of the high value, of the proportion between 5.2 and the maximum stress, the last two bars cannot be recommended for use in concrete structures.

When mild steel bars are stretched beyond their yield point or yield region the cold Work is effected by sliding on slip planes which as' a rule form an angle of about 45. to the direction of the stress. During the sliding these slip planes are blocked, and then offer more resistance to further sliding. This explains the increase in strength by this deformation The first slidings in a steel bar take place within a very narrow region of the bar'with the result that the bar will be thinner and stronger at this point, while the rest of the bar is still'undeformed, and has consequently not received any increase of strength. On further stretching the sliding occurs at another place, until this new slip plane is blocked and so on until the whole of the bar has become cold-worked, and therefore has increased in strength.

Whenever a sliding begins in still undefor..-ed material, the bar is elongated so suddenly that the tensile stress decreases a little. Therefore, at the beginning of I the stress-strain diagram there will be a jagged part, the

ength of which indicates the elongation of the yield region, Fig. 1.. a i

' Therefore no real increase in strength isIobtained by plastic stretching, until the elongation exceeds the jagged part of the stress-strain diagram (Fig. 1). By plastic twisting'the same is the case for the outer fibres of the bar, andif the distant (right) end of the jagged part of the diagram corresponds to an elongation of 4% this corresponds to a twisting of 100 (1. according to calculations, or about d. according to experiment. First when the cold work exceeds the yield region a real and uniform strengthening of the material takes place and therefore the degree of cold-work must exceed this value before strengthening really takes place. To gain a practical increase in strength it is necessary to carry out a coldworking with a degree of deformation exceeding this value about one fourth of the elongation of the yield point or about 2%. 7

, For the same degree of cold work this would give different numbers of turns for bars of difierent diameters. To use the same indication for the same degree of cold work for all bars the length of one turn is expressed'in diameters of the bar. For instance 20 d. means that a generatrix of the cylindrical steel ,bar has received the shape of a screw line with one winding at a length of 20 times the diameter of the bar. 1

7 The qualification of a cold-worked reinforcing bar can in general be judged on the basis of two main properties, which shall ensure against sudden unexpected failure.

The two properties are:

l. The uniform elongation, which is a part of the elongation at fracture, namely the elongation at maximum stress just before the reduction of area sets in. This elongation is called uniform because it is uniformly distributed along the test piece (contrary to the local extension at the position of fracture). The uniform elongation, expressed as a percentage of the original gauge length, is independent of the gauge length, contrary to the elongation at fracture, which depends very much on the gauge length on account of the local reduction of area and the local extension at the position of fracture. The

uniform elongation shall amount to a certain percentage,

for instance at least 4% as required in the Danish standards.

2. The ratio of yield stress to maximum stress. .This ratio must not exceed a certainamount, f. inst. 0.88 as. required in the Danish standards. .As cold-worked bars have often no true yield stress, the stresscausing a per-' manent elongation of 0.2% is conventionally substituted for. the yield stress; i

When a steel is subjected to a cold twisting, the yield point and in particular also the maximum strength increase, but at the same time the uniform elongation will unfortunately decrease. This fact sets narrow limits to the degree of twisting in be discontinued before the first of the above mentioned 7 two conditions is reached.

If, however, a steel is subjected only to a cold stretching, the uniform elongation would only decrease to a slight extent.

Unfortunately the maximum strength will not beincreased to the same extent as the yield point,'and therefore the cold stretching has to be discontinued, when the second of the above mentioned'two' conditions is being reached. When now applying the method according. to the present invention by both twisting and stretching the steel in the cold it will be possible toadjust the con sequent strength improvements-insuch a way that the said above mentioned two conditions will be reached at the same time. It will thus be possible to control the, strengthening in such'a way, that a maximum of strengtheningis obtained satisfyingsimultaneously the-claims of security of the standards. a 7

As before mentioned -Figures 5-8 are diagrams in which some characteristiccold treatments according to the invention are indicated.

V The abscissaeindicatethe degree of twisting, and are" plotted in terms of dwhere d equalsdiameter other. The ordinates indicate the. elongation ;of the bar in percent. 7 p A All experimental values given below are obtained with normal miid steel if no remarks are made about the quality of the steel. Most of the cold-worked bars are cold-worked untila uniform felongation of about "5% is .obtained, and for comparison a twisted steel bar coldthat the twisting process must.

42 kg./rnn1. a tensile strength of 56 kg./mm. and an endurance limit of 36.5 'kg./mm.

By cold stretching a bar until it gets a permanent strain of about 9%, and thereafter cold-twisting such bar until it receives a permanent twist of 14 d., a yield point of 46.4 kg./mm. can be obtained at a uniform elongation of 6%, see line 19 of Fig. 5. V a 7 By cold stretching a bar of alloyed steel by pulling until it receives a permanent strain of 6% and thereaftercoldtwisting to 15 d. the yield point of the bar is increased to 61.8 kg./mm. and the tensile strength to 74.1 kg./mm. and having uniform elongation of 6.5%, see line 20 of Fig. 5. By stretching a mild steel to give the same a permanent strain of 9% and thereafter twisting it to 11 d.

the yield point is increased to 50.0 kg./mm. see line 21 of Fig. 5. i I

.The bar can also be twisted, before it is stretched. By for instance twisting to about 20 d. and thereafteri' stretching to a permanent strain about 3V2%, the yield point is increased to 55.2 kg./mrn. see line 26 of Fig. 5, where by the proportion between the yield point and the tensile strength is increased to the dangerous value of 99.8%. By twisting a steel bar to 17 /2 d. and then stretching it to a strain of 4% the yield point is increased to 56.2 kg./mm. see line 25 of Fig. 5. By twisting 'a mild steel to 18 d. and thereafter stretching to 1.2% the yield point is increased to 53.2 kg./mm. see line 27 of Fig. 5. V

The steel bar could be made subject to' several successive twisting and stretching procedures. For instance it could be subjected to the following cold-work. First a stretching so as to obtain a permanent strain of about 4%, then a twisting to 25 d., then a stretching to a total strain of 9% and finally a twisting to 12 /2 d., see line 30 of Fig. 6. By this cold-working the yield point can be raised to kgjmmi i The first step of the cold work could be a twisting. For instance first twisting to obtain apermanent twist of 75 d., then stretching to 9% ing to 13 /2 d., see line'33 of Fig. 6, whereby the yield point of the steel is increased to 46.4 kg./mm. Because of the twisting, the undesired horizontal part of the stress-strain diagram will be considerably reduced, possibly it will quite disappear, but a following stretching will make it appear again. By excessive twisting after stretching of the steel bar beyond the yield region, care must be taken, as otherl the said stretching wise a heavy decrease in ductility as Well as a lowering of the endurance limit may occur, which results in socalled overworking brittleness. V

The risk 'of this however may be avoided if the procedure which produces a permanent twist in the bar is carried on simultaneously with the procedure by which the bar is stretched beyond its yield region and is given a permanent elongation.

As a matter of fact, it has been shown by effecting during simultaneous twisting that. a considerable increase of the strength properties of the bar may be achieved, even by much less twisting than by a stretching succeeded by a twisting, and therefore the cold-work may be suspended in time before the risk of.

over-working brittleness arises.

By carrying on the stretching and twisting procedure simultaneously, there is not only a saving in time but also a saving of more than 25% of the power required. j A steel bar was twisted about 14 /2 d. and. simultaneously stretched to produce a permanent strain of about 9%, see line 36 of .Fig. 6. This raised t-he'yield point to 51.4 kg./mm.

Another steel bar was first simultaneously twisted to about 75 d} then' stretched to about 7.9% and simultaneously twisted to l8 d. This treatment raised the yield point to 46.1 kg./mm. Finally this bar was subjected to another simultaneous stretch and finally a twiststretched to about .4% and ing and twisting operation giving a total permanent strain to the bar of 8% and a permanent twist of 14 d., see line 39 of Fig. 6. This further treatment raised the yield point to 50.0 kg./mm.

In some cases it might be advantageous to let the stretching decrease during the last part of the cold working. A steel bar was first elongated 7.8% and simultaneously twisted 20 d., then die twisting process was continued at the same speed, namely 40 revolutions per minute and strain was increased to 8%, see line 42 in Fig. 6. The yield point of this bar was raised to 52.7 kg/ mm?.

A steel bar was first twisted to 75 d. and then twisted to 12 /2 d. and simultaneously stretched until elongated 6%, see line 45 in Fig. 6. This raised the yield point to 47.7 kg./mm.

In another instance a bar was first simultaneously stretched and twisted; this simultaneous stretching and twisting giving to the bar a permanent strain of 9% and a permanent twist of 15 d. Finally it was twisted to 11 d., see line 47 in Fig. 7. The yield point of this bar was raised to 54.2 kg./mm.

A stretching can also be undertaken before the simultaneous stretching and twisting. For example, one bar was stretched until elongated 4% and thereafter was simultaneously stretched and twisted until the strain was increased to and a permanent twist of d. was obtained, see line 50 of Fig. 7.

The yield point of such bar was raised to 50.2 kg./mm. The combined operation can be carried out before the pure stretching operation. In one such case a bar was simultaneously stretched and twisted until it was elongated 4% and twisted d. Thereafter the bar was stretched to elongate the same 9% and then twisted 10 d. (line 52, Fig. 7). By this treatment, the yield point of the bar was raised to 54.6 kg./mm. the tensile strength to 64.5 kg/mm. and a uniform elongation of 6% was obtained. In this instance the proportion between the yield point and the tensile strength has the favourable value of 84.7% and the endurance limit is 48.3 kg./mm.

Favourable results have furthermore been obtained by subjecting steel bars to three consecutive cold treatments namely a cold stretching, a cold twisting and a simultaneous cold stretching and cold twisting.

Such a coil worked alloyed steel had its yield point raised to 63.2 kg./mm.

When the three cold workings follow one another, you

can first stretch to a strain of 4% and twist 75 d., and thereafter it is twisted to 14 d. and simultaneously stretched until its total strain equals 10%, see line of Fig. 8.

Another alloy steel was first elongated 4% by stretching, then simultaneously stretched and twisted, the total strain then amounting to 10% and the twisting to 17 /2 d. The bar was then twisted to 14 d. (see line 57, Fig. 8). This steel had the yield point raised to 63.4 kgJmmF, at a uniform elongation of 7%.

A mild steel bar was first twisted d., then stretched to obtain 4% strain and then by a simultaneous stretching and twisting the total strain was brought up to 9% and the permanent twist to 12% d., see line 59 of Fig. 8. This steel had its yield point raised to 50.4 kg/mmfi.

By an experiment a steel bar was first twisted 75 d. and then elongated 7.8% and twisted to 14 d. by simultaneous stretching and twisting and then stretching slightly until the total strain reached 8%, see line 61 of Fig. 8, the yield point was raised to 55.1 kg./mm. and the tensile stress to 60.3 kg./mm. whereby the ratio of yield stress to maximum stress obtained the value of 91.4%.

From the above mentioned examples it will be seen that by applying the method according to the invention the strength properties of the final product can be increased 15-25% beyond the strength properties which are obtainable by any cold treatment hitherto known, it all of course being presumed that the two conditions with regard to uniform elongation and ratio of yarn stress to maximum stress are complied with.

What I claim and desire to secure by Letters Patent is:

In making reinforcing members for concrete structures from hot-rolled steel bars, the method of cold-working such bars by plastic stretching to obtain a uniform plastic strain and a uniform increase in strength throughout their cross-section, and by plastic twisting to obtain an increase in strength chiefly in the outer fibres of the bar, said method comprising pulling such bars until there is a 4% to 9% uniformly distributed elongation of the hotrolled steel bar and a permanent twist of about 14.5 d.

References Cited in the file of this patent UNITED STATES PATENTS 

